tcp: Decrement tcp_md5_needed static branch

[ Upstream commit 6f6e860 ]

In case of early freeing an unwanted TCP-MD5 key on TCP-AO connect(),
md5sig_info is freed right away (and set to NULL). Later, at
the moment of socket destruction, the static branch counter
is not getting decremented.

Add a missing decrement for TCP-MD5 static branch.

Reported-by: Qihang <q.h.hack.winter@gmail.com>
Fixes: 0aadc73 ("net/tcp: Prevent TCP-MD5 with TCP-AO being set")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20260625-tcp-md5-connect-v3-3-1fd313d6c1e0@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: 0x7f454c46

net/ipv4/tcp_output.c

@@ -4124,8 +4124,8 @@ int tcp_connect(struct sock *sk)
 			tcp_clear_md5_list(sk);
 			md5sig = rcu_replace_pointer(tp->md5sig_info, NULL,
 						     lockdep_sock_is_held(sk));
-			if (md5sig)
-				kfree_rcu(md5sig, rcu);
+			kfree_rcu(md5sig, rcu);
+			static_branch_slow_dec_deferred(&tcp_md5_needed);
 		}
 	}
 #endif